Though present in natural settings at minute quantities, ethylene oxide was first synthesized in a laboratory setting in 1859 by French chemist Charles-Adolphe Wurtz using the so-called “chlorohydrin” process. However, the usefulness of ethylene oxide as an industrial chemical was not fully understood in Wurtz's time; and so industrial production of ethylene oxide using the chlorohydrin process did not begin until the eve of the First World War due at least in part to the rapid increase in demand for ethylene glycol (of which ethylene oxide is an intermediate) as an antifreeze for use in the rapidly growing automobile market. Even then, the chlorohydrin process produced ethylene oxide in relatively small quantities and was highly uneconomical.
The chlorohydrin process was eventually supplanted by another process, the direct catalytic oxidation of ethylene with oxygen, the result of a second breakthrough in ethylene oxide synthesis, discovered in 1931 by another French chemist Theodore Lefort. Lefort used a solid silver catalyst with a gas phase feed that included ethylene and utilized air as a source of oxygen.
In the eighty years since the development of the direct oxidation method, the production of ethylene oxide has increased so significantly that today it is one of the largest volume products of the chemicals industry, accounting, by some estimates, for as much as half of the total value of organic chemicals produced by heterogeneous oxidation. Worldwide production in the year 2010 was about 22 million tons. About seventy percent of the ethylene oxide produced is further processed into ethylene glycol; about twenty percent of manufactured ethylene oxide is converted to other ethylene oxide derivatives and only a relatively small amount of ethylene oxide is used directly in applications such as vapor sterilization.
The growth in the production of ethylene oxide has been accompanied by continued intensive research on ethylene oxide catalysis and processing, which remains a subject of fascination for researchers in both industry and academia. Of particular interest in recent years has been the proper operating and processing parameters for the production of ethylene oxide using so-called “high selectivity catalysts”, that is Ag-based epoxidation catalysts that contain small amounts of “promoting” elements such as rhenium and cesium.
With respect to these Re-containing catalysts, there has been considerable interest in determining the optimum conditioning or start-up conditions, since Re-containing catalysts require a conditioning period to maximize selectivity. Without this conditioning or “initiation” procedure, Re-containing, high-selectivity catalysts will not exhibit higher selectivity, but will perform like conventional “high activity” catalysts.
These conditioning procedures are often directed to ensuring the catalyst has a performance-enhancing amount of chloride. The presence of chloride plays a key role in maintaining the catalyst's selectivity—the efficiency of the partial oxidation of ethylene to ethylene oxide. This is especially the case with respect to rhenium-containing, high selectivity catalysts, which are very dependent on the presence of chlorides to achieve optimal performance. Examples of such procedures were previously disclosed in U.S. Pat. No. 4,874,879 to Lauritzen et al. and U.S. Pat. No. 5,155,242 to Shanker et al., which disclose start-up processes in which a Re-containing catalyst is pre-chlorinated prior to the introduction of oxygen into the feed and the catalyst is allowed to “pre-soak” in the presence of chloride at a temperature below that of the operating temperature. While some improvement in overall catalyst performance has been reported using these prior art methods, the pre-soaking and conditioning nonetheless impose a substantial delay before normal ethylene oxide production can begin after oxygen is added into the feed. This delay in production may either partially or entirely negate the benefit of increased selectivity performance of the catalyst.
Temperature is also an important aspect of conditioning—as shown, for example, in the proposed start-up process disclosed in U.S. Pat. No. 7,102,022 to Evans et al., which discloses contacting a Re-containing catalyst bed with a feed comprising oxygen and holding the temperature of the catalyst bed above 260° C. for a period of time of up to 150 hours. Again, while some improvement in catalyst performance may be obtained by this prior art method, there are also inherent disadvantages to this process, notably the high temperatures required during start-up.
Thus, the treatment methods for activating a Re-containing epoxidation catalyst disclosed in the aforementioned prior publications may provide some improvement in catalyst performance, but also have a number of deficiencies. However, given the improvement that an optimized activation process can impart to the selectivity of a Re-containing epoxidation catalyst, the full range of activation processes have not been fully explored. For these reasons there is a continuing need in the art for an improved conditioning procedure for use in olefin epoxidation.